The invention relates to an optical arrangement, in particular to a microlithographic projection printing installation, in particular having a slot-shaped image field or rotationally non-symmetrical illumination, comprising a light source which emits radiation, and an optical element which is heated by being acted upon by the radiation, and a supply apparatus for gas for tempering the optical element.
The imaging quality of such an optical arrangement is often impaired by rotationally non-symmetrical image defects. Such image defects arise, for example, not only as a result of rotationally non-symmetrical light-induced heating of the optical element but also as a result of other light-induced effects, such as e.g. compaction, which lead to a corresponding, rotationally non-symmetrical expansion and/or refractive index distribution in the optical element. When high imaging quality is required, as it is in particular for microlithographic projection printing processes, the described light-induced image defects cannot be tolerated and therefore have to be avoided or corrected.
From U.S. Pat. No. 5,920,377 A an optical arrangement of the type described initially is known, in which the temperature distribution in an optical system is influenced by a gas supply. For said purpose, the volumetric flow is given a spatial distribution which is adapted to the intensity distribution of the radiation. To said end, the housing for the optical system is subdivided into a plurality of gas flow zones, each with a separate gas supply. Selection of the spatial distribution of the volumetric flow is effected in U.S. Pat. No. 5,920,377 A by means of the configuration of the gas flow zones. As the thus influenceable thermal coupling of the optical elements of the optical system to the various gas flows is effected via their peripheral surface, precise adjustment of a specific temperature distribution for compensating image defects is possible only to a very limited extent. In addition, control of the flow conditions in the gas flow zones is problematical. It is impossible to rule out the formation in the gas flow zones of dead volumes, in which the gas is substantially stationary and so in an undesirable manner no heat transport occurs.
The object of the present invention is therefore to develop an optical arrangement of the type described initially in such a way that an improved adjustment of a specific temperature distribution in the optical element may be achieved.
Said object is achieved according to the invention in that the supply apparatus comprises at least one supply line and at least one gas directing device, which is aligned relative to the optical element and controllable in such a way that the gas is directed by the gas guiding device as a free flow towards the optical element and the volumetric flow of the exiting gas has a magnitude and spatial distribution, which are adapted to the intensity distribution of the radiation.
By means of such a gas directing device the tempering of the gas-exposed region of the optical element may be adapted well to the shape of the light-induced image defect and the temperature distribution in the optical element may be purposefully selected. The gas directing device in the present case causes the gas to flow directly against the optical element so that, by virtue of the thermal coupling of the latter to the gas flow, effective tempering is provided. Dead volumes, from which gas is not removed, are avoided particularly in the regions of the optical element which are to be tempered.
In the present case, thermal coupling to the gas flowing past leads e.g. to extensive compensation of the asymmetrical heating caused by the light absorption, resulting in a rotationally symmetrical temperature distribution. Image defects based on thermal expansion of the material of the optical element are thereby avoided. However, by virtue of a purposeful flow against the optical element it is also possible to compensate additional image defects caused, for example, by compaction and the resulting change of refractive index. For said purpose, by means of the flow of gas against the optical element an asymmetrical temperature distribution is deliberately produced in the latter, wherein the consequent changes of the imaging properties of the optical element as a result of the asymmetrical expansion of the material of the optical element and the image defects caused e.g. by compaction compensate one another in such a way that, on the whole, selected imaging properties of the optical element arise.
A throttle valve may be disposed in at least one supply line for the gas directing device. By said means the magnitude of the volumetric flow is easily adaptable to the requirements for tempering of the optical element.
The gas directing device may be formed by at least one nozzle, which is connected by the supply line to a gas source. A nozzle enables a precise flow against a region of the optical element which is to be tempered.
Preferentially a plurality of gas directing devices are provided, with each of which a throttle valve in a supply line section is associated. This enables an adaptation to an irradiation with projection light, the intensity distribution of which varies over the irradiated area of the optical element. For example, given use of a cooling gas, the flow against regions which are subjected to increased light-induced heating may be correspondingly greater.
For the gas directing device an adjustable holding device may be provided. This enables purposeful alignment of the gas directing device relative to the optical element.
The holding device may comprise an adjusting device for adjusting the axial position of the gas directing device relative to the optical element. It is therefore possible to adjust the size of the zone of impingement of the gas flow exiting from the gas directing device in the direction of the optical element.
Alternatively or additionally the holding device may comprise an adjusting device for adjusting the inclination of the gas directing device relative to the optical element. By means of an inclination adjustment both the position and the shape of the respective impingement zone of a gas directing device are adjustable relative to the optical element.
A control device with a communication link to the at least one throttle valve is preferentially provided for selecting a volumetric flow of gas in the gas directing device. This enables automatic and precise adjustment of the volumetric flow of the gas provided for tempering.
Said control device may have a communication link to the light source for receiving a signal corresponding to the light output of the light source, wherein selection of the volumetric flow of gas is effected by the control device in dependence upon the transmitted signal of the light source. This allows the gas tempering to be adapted to the projection light output used in each case.
Alternatively or additionally a sensor arrangement with a communication link to the control device is provided for monitoring the imaging quality of the optical element and/or of the optical arrangement, wherein selection of the volumetric flow of gas is effected by the control device in dependence upon the transmitted signal data of the sensor arrangement. This enables a regulation of the gas supply, whereby the optical element is tempered by the gas supply in the regions selected by the control device on the basis of the signal data relating to the imaging quality. Such regulation allows, for example, image correction of a system of optical elements. In said case, by means of the gas tempering of an optical element the imaging properties of the latter are over-compensated in such a way that, despite the light-induced image defects of the other optical elements, on the whole an image-corrected optical arrangement is achieved.
The sensor arrangement may comprise a CCD array. By means of such a sensor arrangement a precise acquisition of the imaging properties of the optical element and/or of the optical arrangement is possible.
The gas directing device is preferentially part of a sweeping device for the optical element and/or optical arrangement. The sweeping and tempering functions are therefore combined.
In a preferred form of construction, a thermostatted tempering device is provided in the supply line. This enables an exact temperature selection of the gas flow so that the tempering possible with a gas flow of a preselected volumetric flow may be exactly determined.